Electric cables may be used for both direct current (DC) or alternating current (AC) transmission or distribution.
Cables for power transmission or distribution at medium or high voltage generally are provided with a metallic electric conductor (usually aluminium or copper) surrounded—from the radially innermost layer to the radially outermost layer—with an inner semiconductive layer, an insulating layer and an outer semiconductive layer respectively.
In the present description, the term “medium voltage” is used to refer to a voltage typically from about 1 kV to about 30 kV and the term “high voltage” refers to a voltage above 30 kV.
Telecommunication cables typically comprises at least one telecommunication conductor, e.g. an optical fibre contained in a tube optionally together with water swellable elements in form of gel, yarns or powder. Depending on the size and on the scope intended for the telecommunication cable, the tube is in turn contained in a sheath.
As “cable core” it is herein meant the portion of the electric or telecommunication cable comprising the electric or telecommunication conductor and the adjacent cable elements.
Many problems can arise due to water contacting the cable core.
In the case of electric cables, conductor and insulating layer are particularly sensitive to such a contact. Water can induce corrosion of aluminium conductors and cause the formation of gaseous hydrogen in the insulating layer, the so-called “water-treeing” phenomenon can impair the dielectric strength and bring to cable perforation during operation.
In the case of telecommunication cables, the optical fibre telecommunication conductors can undergo attenuation phenomena in contact with water. Also, water can reach and degrade closure or other termination device and/or can damage electronics mounted within the closure or other termination device.
Thus, the penetration of water into cables, and stagnation therein, is an event that should be avoided as it spoils the cable reliability.
After manufacturing, cables are usually stored and shipped with protection caps on their heads.
However, the penetration and stagnation of water within the cable core can occur despite the above precautions. In particular, water penetration and stagnation cannot be excluded during installation, for example due to negligence of the installing personnel.
Water diffused into a cable via cable head can be eliminated by, for example, blowing nitrogen. The problem is when the water penetration and stagnation in a cable is not readily visible because, for example, the cable head dried before inspection. In such instance, water can have caused damages to the cable core and can even be still present in the cable in a position distant from the cable head.
GB 1,420,365 relates to an electric cable, which is self-sealing upon penetration by water, comprising one or more insulated conductors located within a cable sheath, said cable sheath accommodating a composition consisting of a material or a mixture of materials which significantly changes colour when contacted by water, together with a material or a mixture of materials which swells and optionally evolves a gas when in contact with water.
A mixture of materials, which change colours, comprises potassium ferrocyanide and ammonium iron(III) sulfate. The dry mixture is white/yellow but following contact with water it turns to an intense blue (for example, Prussian blue). Alternatively, materials which when dry display only little colour or no colour at all, but which yield an intensely coloured aqueous solution may alternatively be used (for example, Astra diamond green).
The Applicant noted that the materials or mixtures disclosed above react as soon as they come into contact with water. Such fast reactions are not desired because could generate useless alarm. As a fact, brief water washings do not substantially harm the cable integrity.
High sensitivity of the material to humidity or moisture is equally undesirable as it could give place to unwanted reaction also at the manufacturing stage.